​The molding properties of sheet material for blister technology.

The molding properties of sheet material for blister technology.

Sheet used for molding process, either single layer or multi-layer composite, must have the following properties:

First, plastic memory, that is, when stretching the softening of the sheet, both the tendency to tighten the resistance to tension, but also as far as possible the tendency to stretch uniformly. This characteristic can make the products that have been molded if reheated to the original molding temperature, it will return to the original flat sheet shape. This characteristic has an important effect on the stretching of the molding process.

Second, hot stretching, that is, the sheet can be stretched when heated, this characteristic has a great impact on the shape and quality of the product. Some can stretch 15% to 20%, and some can even stretch to 500% to 600%.

Third, the thermal strength, that is, the heating of the softened sheet as long as the slightest pressure, will form a clear outline on the mold. On the contrary, if you need too much pressure to mold, and vacuum vacuum molding to provide pour pressure difference is limited to certain subtle patterns will be difficult to show.

Fourth, the molding temperature, that is, the molding sheet needs to have suitable for a certain processing temperature range. Both in its heat softening temperature, easy to mold, and its melting temperature has a certain distance, the molding temperature range is wider; can not be only in a smaller specific temperature range of molding, high or low temperature, molding is easy to tear, melting collapse and other phenomena.

When heating thermoplastic molding materials, amorphous and partially crystalline thermoplastics produce differences. For amorphous thermoplastics, vacuum vacuum molding is possible when the temperature is raised to its glass transition temperature (softening point). The amount of heat required depends on the type of material being molded and the processing method used, with vacuum molding requiring higher temperatures than pressure molding.

Partially crystallized thermoplastics are never fully crystallized and are divided into amorphous and crystalline zones. When the degree of crystallization is low, the material can be vacuum formed at temperatures below the crystalline melt zone, and when the degree of crystallization is high, higher molding temperatures are required.

How does this phenomenon with thermoplastics affect vacuum vacuum molding processing? If a partially crystallized thermoplastic is used to work at high temperatures, or if the part itself needs to be sterilized at high temperatures, it needs to be subjected to thermal-stress conditions without deformation, which requires that the partially crystallized areas be completely molten during thermal processing, i.e., that the molding material needs to be selected for a sufficiently high vacuum pickup molding temperature.

As an example, partially crystallized polypropylene thermoformed cups are used to illustrate the effect of different vacuum vacuum forming temperatures on the appearance of the molded product and the subsequent autoclaving process. Polypropylene's crystalline melting zone is roughly 158 ~ 165 ° C, when the vacuum blister molding temperature is lower than 158 ° C, the crystalline zone will not be completely melted, they are like a number of small plastic blocks, in the molding process to produce deformation, but still retained in the form of solid in the amorphous melt. As soon as it is heated again (e.g. sterilized at high temperatures), the cup starts to deform, this is because the stresses in the crystalline zone want to return to its original shape.

Vacuum molding temperature of 130 ~ 135 ℃, the heating time should be as short as possible, as little as possible crystallization occurs, so that the material has the desired deformability. High crystallization is not conducive to the accuracy of the product. Vacuum suction molding mold must be heated to 170 ℃. During the vacuum suction molding process (3.5 to 4s for 0.6 to 0.7mm thick sheet), the material can obtain a crystallinity of 25% to 30%. After molding, the product is cooled in another mold at 60°C. The maximum long-term working temperature of CPET vacuum forming products is 220°C, but it must be remembered that it can be used at such high temperatures and the stability of the product depends on the degree of crystallinity. For example, containers made of CPET have a crystallinity of only 25% to 30%.

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